The invention relates in general to coordinate measurement machines (CMMs) and in particular to a method and system for assisting a user taking measurements using a CMM.
The manufacturing/industrial marketplace took on a new face during the 1980's with the introduction of computer-aided design (CAD) and computer-aided manufacturing (CAM). While CAD allowed engineers to produce 3-D images in the front end of the design process, which shortened the production cycle and led to tremendous gains in productivity, CAM software and equipment increased the efficiency and quality of machined single parts. In essence, these new technologies revolutionized the marketplace by increasing productivity, improving quality, and reducing costs.
Despite these technological advances in design and manufacturing, something important was missing from the production cycle: a highly accurate, efficient, and convenient measurement methodology for ensuring that the products and components—both on and off the production line—met the original CAD specifications. The design process, with the help of CAD, had become innovative and sophisticated; so too, had the machining process through CAM. Yet measuring the assemblies made of these parts against the CAD model, for the most part, has continued to remain unwieldy, expensive, and unreliable.
Traditionally, the measurement and quality inspection function in the manufacturing process has been time-consuming and limited in size, scope, and effectiveness for a number of reasons. Manual measurement tools, such as calipers and scales may be slow, imprecise, and always one-dimensional. Analog test fixtures are costly and inflexible. And standard stationary CMMs while providing a high degree of precision, are generally located in quality control labs or inspection departments at a distance from the manufacturing floor. Parts must be removed one at a time and transported to the lab for inspection. As a result, CMMs measure only small, readily-moved subassemblies and components—which often translates into significant “down time” for the production line. In essence, traditional measurement techniques—also known as metrology—have lagged far behind in the technological advance of the production process.
The CAD/CAM and metrology markets, as well as a worldwide emphasis on quality in all aspects of the manufacturing process, are driving the need for an extension of the CAD/CAM techniques, which the inventor refers to as computer-aided manufacturing measurement. This last phase of the CAD revolution has remained incomplete because of the significant technical demands for adaptive measurement hardware and usable CAD-based measurement software for the difficult manufacturing environment. Therefore, there exists a need to take conventional metrology from a single-parts-only, high-level precision testing methodology to a whole products, intermediate-level precision measurement system at every step of the manufacturing process at any location on the factory floor. Measurements of part dimensions and/or characteristics may be made on the production floor to determine compliance with design specifications and ensure quality.
Previously, inspections to determine compliance with design specifications required highly skilled inspection personnel who could understand and interpret the associated documents. Geometric dimensioning and tolerancing (GD&T) requirements further complicated the inspection process.
GD&T is a mathematical language using specific terms and symbols to define a part's size, form, orientation and location of features based on how the part will function in the final product. Therefore, GD&T allows a designer to define a part's dimensions based on the part's final usage. In other words, GD&T is essentially a language used for “functional dimensioning.” Using GD&T to define a part allows greater design freedom and lower manufacturing costs. Without GD&T, a designer might arbitrarily and unknowingly define a part's tolerances too tight for cost effective manufacturing.
FIG. 2 shows a chart with various GD&T symbols and the corresponding characteristic. GD&T data is typically represented in a format known as a document feature control frame shown by way of example in FIG. 3. A standard convention exists for the creation of the document feature control frame. The document feature control frame is typically found on a document such as a design drawing, specification or part of a digital definition of a part. The standard convention stipulates a number of tolerances and data features, which should be used in determining the acceptability of the corresponding part. The internationally accepted standard is ASME Y14.5M-1994 (American Society of Mechanical Engineers) or ISO 1101 (International Organization for Standardization). Many hours of training are required to become proficient in applying and using tolerance standards such as ASME Y14.5M-1994/ISO 1101.
For example, to inspect an automobile quarter panel that attaches to a mating part with five slots, all surfaces may require tolerancing with respect to the slots. Typically, the inspector must first review the document corresponding to the panel, and any GD&T data thereon. Next, the inspector must take measurements and perform complex mathematical calculations based on the data in the document feature control frame 20. Thus, determining the appropriate measurements and calculations during an inspection required technical skill in addition to GD&T training. Usually, only highly trained individuals, such as engineers and skilled technicians with GD&T training, could interpret tolerance requirements and plan an accurate approach to measuring and inspecting the corresponding part.
Even in situatons where a computer is assisting in the calculations, a thorough understanding of GD&T is necessary to interpret the symbols and numbers provided on a formal engineering drawing containing GD&T information. Prior automated systems did not track datum designations and relied on the user to select part features corresponding to datums identified in the formal drawing from a generic list of measured features, making the tolerance evaluation unnecessarily tedious and prone to mistakes.
Therefore, there exists a need for a system and method for assisting a user, such as a machinist or a person without technical or GD&T training, in taking measurements using a CMM.